A polyester is generally produced by direct esterification of an aromatic dicarboxylic acid and glycol, or by ester exchange reaction of an alkyl ester of aromatic dicarboxylic acid and glycol to give a glycol ester and/or an oligomer followed by heating them with stirring under high vacuum to effect polycondensation.
U.S. Pat. No. 2,597,643 discloses that polymerization of polyester can be carried out in a solvent. In such reaction, the polymerization is, however, carried out in a solution which can completely dissolve a resulting polyester, and hence, the resulting polyester has a very low intrinsic viscosity of 0.4-0.7.
There has been widely and industrially used a process wherein polycondensation is carried out by heating and stirring under a high vacuum. This process, however, requires certain vacuum apparatus to keep high vacuum and high power for stirring of material having high viscosity.
Further, since materials for industry use, such as a tire cord and the like, require higher physical properties, there is used a polyester having a high molecular weight. However, it is difficult to stir such a polyester having a higher molecular weight, and hence, a polymer obtained by melt polycondensation is generally subjected to further solid phase polymerization under vacuum or an inert gas flow for a long term of time. In the latter process, however, a polyester having intrinsic viscosity of, at most, 1.0 to 1.5 is usually prepared. In addition, it is difficult to completely dissolve the resulting polyester in a solvent.
On the other hand, it has been reported that a polymer is pulverized in fine powder and subjected to solid phase polymerization to obtain a polyester having a molecular weight of about 120,000 (Cryogenic Properties of Polymers, 249, Dekker). However, in this case, it has been also reported that the resulting polyester is partially insoluble in a solvent used for measuring viscosity, and it is assumed that the molecular weight of the polyester is apparently increased due to a crosslinking reaction.